Many applications incorporate radio frequency identification (RFID) tags. RFID tags employ reflected energy originally transmitted from an RFID reader, and do not generate RF energy. Some Real Time Location Systems (RTLSs) track objects by associated RFID tags. For individuals, a badge is used for tracking in environments such as health-care facilities, warehouses, and other areas where location is important. These RFID personnel badges communicate with fixed or hand-held readers. These devices employ a combination of antennas and electronics. They provide structures to support and protect the antennas and electronics, and to mount or attach them to objects. In many applications size, shape and mechanical properties such as flexibility, are important but impeded. Bulky materials and construction add undue thickness and stiffness to devices. These devices require adequate electrical connections, mechanical support, and appropriate positioning of components such as connectors and antennas. Structures for these purposes can add complexity, thickness, inflexibility and cost to the RFID device.
Additionally, RFID systems operate over various frequencies from high-frequency (HF) through super-high-frequency (SHF). Multiple operating issues arise in these ranges. While the performance of an RFID tag operating in the HF band may be less affected by the tag's proximity to the human body, a typical worn device is approximately 10 cm in length, including the antenna. This is a small fraction of a wavelength for the lower frequencies. Antennas which are a small fraction of a wavelength in linear dimensions are very inefficient radiators and receptors. As a result, the useful operating range for the HF band can be just a few inches from the reader antenna, significantly limiting the usefulness of HF tags. RFID systems operating at higher frequencies, however, may provide longer ranges in part because the shorter wavelength is more comparable to the antenna dimensions. This dimension match improves efficiency. However, compared to the HF band, signals at these higher frequencies are much more strongly affected by obstacles and materials in the immediate environment of the antenna due to the shorter wavelengths. Furthermore, antennas operating on or adjacent to the human body will be severely detuned and possibly rendered inoperable. Thus, the usability of these antennas in identification devices with RFID capability is very limited. When on or near the surface of a human body, the reactive near fields are influenced by the human tissue and there may be an impedance mismatch between the antenna and connected circuits, resulting in poor overall efficiency. This mismatch may detune the antenna and reduce the energy radiated away from the body, further impairing performance.
Another characteristic of RFID systems is that the performance of such systems is governed by the Radar Equation (1).
                              P          r                =                                            P              t                        ⁢                          G              T                        ⁢                          A              r                        ⁢            σ            ⁢                                                  ⁢                          F              4                                                                          (                                  4                  ⁢                  π                                )                            2                        ⁢                          R              4                                                          (        1        )            
Where Pr=the power returning to the receiving antenna, Pt=the transmitter power, Gt=the gain of the transmitting antenna, Ar=the effective aperture (area) of the receiving antenna, σ=the radar cross section or scattering coefficient of the target, F=the pattern propagation factor, and R=the range
Essentially, this means that the system performance is limited by the fourth power (1/R4) of the distance to the RFID tag. This is due to the fact that the RFID interrogator transmitter power needs to reflect off the RFID tag and be received by the interrogator.
The above problems with RFID systems may result in limited range, difficulty simultaneously tracking multiple proximate tags, and limited information capacity. What is needed is an economical, flexible and wearable device that efficiently communicates information in support of applications such as personnel or equipment location.